Conventionally, a swash plate type compressor is used in systems such as an air conditioning system of an automobile. According to a known swash plate type compressor, the transmission of motive power is carried out, as a swash plate and a piston reciprocate, thereby suctioning, compressing and discharging the gas. The swash plate is usually composed of aluminum or aluminum alloy and makes slideable contact when it rotates with shoes which are composed of iron or light weight ceramics such as alumina. The metal on metal contact at the shoe and swash plate interface can be subject to undue wear and possible seizure of the shoe with the swash plate.
Treatments have been developed to improve the lubricity at the shoe/swash plate interface and lessen the swashplate surface wear. Conventionally swashplates are tin coated to improve lubricity. However, tin is relatively soft and while enhancing lubricity, has less than optimal hardness and durability. To improve the hardness, U.S. Pat. No. 5,056,417, for example, treats the swashplate body with a surface coating layer made of tin and at least one metal selected from the group consisting of copper, nickel, zinc, lead and indium. While any of these five materials are alloyed with tin to improve its wear resistance, none of them are described as also acting to bind the coating to the swashplate. U.S. Pat. No. 5,655,432 treated the swash plate with a cross-linked polyfluoro elastomer bonded directly to the aluminum, a lubricious additive and a load bearing additive. The coating process described adds to manufacturing complexity, and makes it more difficult to hold manufacturing tolerances than with a conventional tin conversion coating. In U.S. Pat. No. 5,864,745, a copper based flame-sprayed material which includes an unmelted structure of an atomized copper alloy powder is disclosed for coating swashplates. The coating may include one or more of lead, tin, phosphorus, aluminum, silver, silicon, manganese, chromium, nickel, and zinc not to exceed 50% of the coating. One drawback of flame sprayed coatings is that often the surface finish produced is too rough, so that post-coating machining is required. U.S. Pat. No. 4,252,618 discloses a electroplating process for tin wherein the bath contains tin ions, an alkali metal hydroxide, and bismuth citrate. This bath is taught to inhibit tin "disease" through the use of the citrate. Electroplating is known to add to the dimension of the electroplated part by the coating thickness, and electroplating requires the use of electric current which adds to the complexity of the coating process.
The present invention discloses a tin--cobalt--bismuth coating formed using conversion coating techniques which overcomes the deficiencies of prior art coatings. It provides a conversion coating which exhibits improved wear resistance and also excellent adhesion to aluminum or aluminum alloy surfaces which experience during use, sliding friction, e.g., surfaces of swashplates, pistons, etc. The coating also retains the high-lubricity of tin on the aluminum swashplate. The metals of the coating diffuse into the aluminum with no net part dimension growth observed.